This project aims to determine the intracellular distribution of diffusible and structural components within axons, dendrites, glia, and synapses. This work is important because of the relationship between the localization and movement of cellular constituents and their role in synaptic transmission. Quantitative x-ray microanalysis of the intracellular distribution of calcium and other elements in parallel fiber/Purkinje cell cerebellar synapses indicates that among all the pre- and postsynaptic organelles only the smooth endoplasmic reticulum accumulates significant calcium under physiological conditions, and that this accumulation occurs from extracellular sources as a function of synaptic activity. The amount of calcium taken up in the cisterns of the Purkinje cell dendritic spines -- a three fold increase from 700 ions to 2100 ions -- is consistent with calcium entry into spines via a ligand-gated ion channel. Complementary new projects using visual system models of neurons and glia have begun to yield data on the identity and activity of intracellular calcium stores. Immunocytochemical studies using frozen sections of actively myelinating peripheral nerve at the electron microscope level have demonstrated the co-localization of the cytoskeletal proteins actin and spectrin and the Schwann cell adhesion molecule MAG in specific cytoplasmic domains of the Schwann cell. These results support a mechanism for myelin sheath formation involving MAG-mediated myelin attachment directed by cytoskeletal motility. Thus, this project continues to provide new information on the detailed relationship between the diffusible and structural components of neurons and glia, and how these regulate neuronal activity.